1. Field of the Invention
The present invention relates to a process for preparing cubic boron nitride. It relates further to a process for preparing magnesium boron nitride which is useful as a starting material for the production of the cubic boron nitride.
2. Description of the Prior Art
Cubic boron nitride, i.e. boron nitride having a cubic crystal structure, is superior in its chemical stability to diamond, although it is inferior in its hardness to diamond. For instance, it is durable in an oxidizing atmosphere at a high temperature, and its reactivity with an iron group element is extremely small. Accordingly, it exhibits substantially superior mechanical properties to those of diamond when used for grinding a heat resistant high strength material containing nickel or cobalt as the basic component, or a high speed steel. Thus, the cubic boron nitride is a quite useful substance.
It has been common to prepare the cubic boron nitride by treating hexagonal boron nitride at a high temperature under high pressure in the presence of a catalyst. As the catalyst, there have been known (1) elements belonging to Groups Ia and IIa, (2) rare earth elements, actinide elements, tin, antimony, lead, etc. (in practice, these elements are used in the form of their nitrides or alloys), and (3) urea and ammonium salts.
However, when the above-mentioned metals or alloys are used as the catalyst, unstable boron compounds and free boron are likely to form as by-products and they tend to be included in the cubic boron nitride crystals thereby obtained. The crystals thereby obtained have drawbacks such that they are blackish and opaque, and the strength of the particles is rather low.
In the case where the above-mentioned nitrides are used as the catalyst, unreacted nitrides will remain in the system, and they are likely to be trapped in the cubic boron nitride. Accordingly, it is thereby difficult to obtain cubic boron nitride crystals having high quality. Besides, the chemical reaction system is rather complicated, and it is difficult to properly set the conditions to obtain the desired crystals.
In the case where the above-mentioned urea or ammonium salts are used as the catalyst, the cubic boron nitride thereby obtained tends to have an extremely small particle size at a level of from 0.1 to 0.5 micron. Its usefulness as grinding particles will thereby be limited. Further, the mechanisms of the chemical reaction and the product formation are not yet been clearly known, and accordingly, it is not possible to set the reaction conditions to increase the particle size.
Other than the methods using such catalysts, it is known to prepare the cubic boron nitride with use of calcium boron nitride as a solvent. The cubic boron nitride thereby obtained has a high quality with a minimal content of impurities, and it is also superior in its mechanical strength. Namely, during the process of conversion of the hexagonal boron nitride to the cubic boron nitride, the calcium boron nitride serves as a solvent for the hexagonal boron nitride, and accordingly, it is possible to crystallize the cubic boron nitride from the liquid phase by the treatment at a temperature sufficiently high to solubilize both reactants, i.e. to establish a co-solubilization condition, and under a pressure required to establish a thermodynamically stable phase of the cubic boron nitride at the temperature.
However, calcium boron nitride (Ca.sub.3 B.sub.2 N.sub.4) has a drawback that it is relatively unstable against moisture. For instance, when exposed in the air, it will be decomposed into a hydroxide by the moisture in the air. Accordingly, in order to maintain its desired function as the solvent, it is necessary to take special cares for its storage or at the time of filling it to the high pressure cell.
Now, with respect to a process for the preparation of magnesium boron nitride useful as a starting material for the production of the cubic boron nitride, it has been reported to form magnesium boron nitride by heating boron nitride and metal magnesium at a temperature of 1150.degree. C. under pressure of 2.5 GP. The magnesium boron nitride obtained by the reported method, has the following X-ray diffraction peaks.
TABLE 1 ______________________________________ Lattice spacings and intensities obtained from the major X-ray diffraction lines of the magnesium boron nitride prepared by the conventional method. Lattice Lattice spacings Intensities spacings Intensities ______________________________________ 7.76 weak 1.68 strong 3.88 weak 1.65 moderate 3.02 moderate 1.57 moderate 2.69 weak 1.55 moderate 2.59 strong 1.49 moderate 2.44 strong 1.46 moderate 2.38 weak 1.32 weak 2.21 strongest 1.31 weak 2.12 strong 1.27 weak 1.94 moderate 1.22 weak 1.89 weak 1.19 weak 1.82 moderate 1.10 weak 1.75 weak ______________________________________ In the above relative intensities, strongest = 100, strong = 70 to 30, moderate = 30 to 10 and weak = 10 to 3.
This method has drawbacks such that high pressure is required and the productivity is poor.